Thanks for the help and ideas guys. It's not the pinion or spur, I have changed several times to make sure it wasn't. I had the pin slipping in my CVDs and fixed it a while back with shoogoo and fiber reinforced tape at the loss of a aluminium front knuckle. I have tore down the car several times. Checked the shims ,gears , driveshaft and anything else I could think of. I think it is either one of two things the front bevel gear is slipping under load (like BritRacer said) or I have mismatched 1.0 version gears and 1.1 version gears
. I just can't pinpoint it at all. Thanks for the help but it looks like I am going to have to throw some money at it and see if that helps.